December 2010

A big system with the potential to dump snow (we’re talking 8-12+ inches possible, but the location remains uncertain) along the East Coast along the I-95 corridor starting late on Christmas Day and going through the 27th. There is still a high uncertainty with the track of this storm, so snow forecasts won’t be that accurate up until within two days of the event.

So keep an eye on the forecast and adjust your travel plans as necessary. There is some potential for this to be a big storm, though I’m still about 50/50 on this event hitting the East Coast hard. As of now, my call for DC/MoCo is 2-4″, which could go up to 3-6″ tomorrow if the models converge on a snowier solution.

I might not get to update the blog until Saturday, as I will be traveling to NY tomorrow, and I’ll be coming back to MD to work on Christmas.

Only highlights this week as I just spent a good amount of time on the January forecast and explaining it in more detail on the American Weather Forums.

—–Highlights:

Rain and snow continue to pummel the West – Heavy rain and snow should continue through Wednesday in the Southwest as more storm systems impact the Pacific Northwest throughout the next week.

Burst of heat in the Southern Plains – Temperatures 8-15+ degrees above normal are expected across the Southern Plains and into the eastern half of the Desert Southwest through most of the work week.

Christmas coastal storm in the East? – A potent storm system will track southeastward from the Midwest into the Mid-Atlantic Christmas Eve into Christmas Day, with the potential to turn northeast and drop snow along the I-95 corridor going into the 26th. Confidence on the I-95 turn is moderate.

Another shot of cold in the East – Behind the potential coastal system, much below normal temperatures are expected to work back into the East Saturday through Monday before temperatures start to slowly warm back up.

UPDATE: The weekend coastal storm has slowed considerably, with a late 25th-27th time frame for the eastern U.S.

A look ahead into what the current status of the atmosphere and mid-range forecast has done to the long term outlook.

The updated January 2011 temperature anomaly forecast:

And for reference the old January forecast (issued 7 October):

The changes:

The lingering blocking pattern is what caused the great change in the temperature forecast. As we move into January, the -NAO/-WPO pattern still exists, but is forecast to weaken as the indices trend towards neutral values (the NAO more so than the WPO). Should this blocking pattern break down completely and transfer into a neutral PNA, neutral WPO and weak +NAO, we will see a much more progressive pattern that will allow the above normal temperatures to work back into the eastern U.S.

My original map included a fully progressive pattern throughout the month of January, with the primary storm track going from the Pac. Northwest into Canada as the subtropical ridge lingered over the Desert Southwest. The more persistent blocking along with the strong -WPO allows the cold air to seep further into the mid-continent, which with plenty of available cold air would lead to drastic changes in the forecast like those seen above.

I would expect to start seeing the effects of the more progressive pattern around mid-January, which is somewhat-aligned with the pattern that the latest Euro weeklies show.

There is a risk for a more persistent blocking pattern or reinvigorated blocking pattern heading further into January, which would result in a cooler eastern half of the nation.

The forecast is almost a repeat of last week for the region, with continued cold, snow in the mountains on Tuesday and maybe some light snow east of the mountains, and coastal system potential towards the weekend.

Temperatures won’t be as brutally cold this week as it was last week, but we can expect to see temperatures about five degrees below normal through most of the week. A weakening low pressure system will move into the region from the Midwest Tuesday into early Wednesday, which is expected to bring snowfall to the windward side of the Appalachians. Along the coast, we might see some snow flurries Tuesday night into Wednesday morning, but accumulating snow is not expected (though some accumulation in the northern parts of the region is possible).

Late Christmas Eve and through Christmas Day (Fri-Sat), the region could be subject to a large snow storm. At this point in time, I am about 50% confident on a coastal storm this weekend. The snow is expected to start off ahead of the system as it glides in from the Midwest and tracks off the coast. After reaching the Atlantic, this system will rapidly intensify, causing snow to wrap around the storm center and enter the region from the northeast. The confidence in this system is a bit higher because of the good model agreement we have been seeing over the past day or two as the GFS model consistently forecasts a coastal storm. Despite this, we could still run into the same problem we saw with this weekend’s system that ended up out to sea. A more progressive and less amplified pattern would allow the storm to move more quickly to the east, which would prevent the storm from bombing out quickly enough to have a major impact on the coastal areas as it moves further into the Atlantic. There could also be some mixing issues in the southern parts of the region initially, but the big snowfall should not come until after the system has moved off the coast. I will have an update on this system Wed/Thu.

Needless to say, this storm could have some major impacts on Christmas plans from Virginia up through the Northeast along the I-95 corridor. Be extra careful traveling this holiday weekend!

As this potential snow storm begins to materialize in the Southeast, it’s time to take a serious look at snowfall predictions. This storm appeared to be going out to sea at the start of this week, then for a couple of days it looked like the I-95 corridor would land a rare epic La Niña snowstorm along the East Coast. Unfortunately for the snow lovers of the East, the models are converging on a system that heads out to sea (OTS), with a quick snow dump over southern Virginia and a wintry mix in the Carolinas before sweeping northeastwards tantalizingly-close to the coast. The coastal areas of New England also have a shot at some snow.

Most of the precipitation will be off the coast, giving the fish a good time while the snow lovers despair over what could have been. At one point, 6-12 inches along the I-95 corridor seemed like a possibility. Now they’ll be lucky to get a dusting. Northern New England could get some backlash snow from this system after the 20th when the system tries to retrograde back to the west.

Here’s an estimate of what this system will put down on the 18th through the 20th (Sat-Mon):

One of the better multi-vortex tornado specimens was captured on film by several chasers, with the seemingly best footage from Verne Carlson that shows 4-5 vortices within a larger tornadic circulation. You can get more information about this storm and about Verne’s chasing on his weather blog.

Skip to the 1:00 mark to watch the incredible dynamics at work!

For some reason there is no audio until 1:59, which is a shame. Still a great video, though!